1. Field of the Invention
The present invention relates to a video converter and a method for converting video data encoded by predetermined encoding means into data encoded by another different encoding means. Particularly, it relates to a video conversion system, a conversion method and a computer program for converting MPEG-2 video data into MPEG-4 video data. Incidentally, MPEG-2 and MPEG-4 are international standards for moving picture encoding.
2. Description of the Related Art
Video contents produced and broadcast by a TV broadcast station, a CATV (cable TV) station, a Web contents provider or the like are generally encoded in MPEG (Moving Picture Experts Group) or the like and stored in digital form in various kinds of recording media such as HDD (Hard Disk Drive) and DVD (Digital Versatile Disk) so that the video contents can be used secondarily. On this occasion, particularly MPEG-2 (Moving Picture Experts Group Phase 2) is used frequently because it is known that MPEG-2 can be used in DVD and HDTV (High Definition Television) and is high in picture quality after encoding.
At present, there is a discussion about a video delivery service for delivering video contents so that the video contents stored in the aforementioned manner can be played back by a PDA (Personal Digital Assistant), a PC (Personal Computer) or the like connected to a cellular phone or the Internet.
It is however substantially difficult to deliver the video contents in the form of MPEG-2 directly because the amount of information (from the order of hundreds of kbps to twenty and several Mbps) in MPEG-2 is larger by several times than the transmission capacity (from the order of tens of kbps to ten and several Mbps) in infrastructure of these circuits. Therefore, for example, a method of once converting MPEG-2 into MPEG-4 (Moving Picture Experts Group Phase 4) higher in information compression rate than MPEG-2 and storing the MPEG-4 again (or delivering the MPEG-4 immediately) may be conceived.
Incidentally, the number of video contents handled by a TV broadcast station, a CATV (cable TV) station, a Web contents provider or the like is enormous. When these video contents are to be processed by the video delivery system, “completion of conversion of a great deal of video contents within a limit time set in advance” is requested for the sake of convenience of a broadcast schedule. Particularly it is important to achieve the system support so that a predetermined conversion process can be executed to the last without failure even in the case where a request of conversion with a load too large (to complete conversion within the limit time) is set.
In the related art, there is a system of converting MPEG-2 video contents into MPEG-4 video contents (e.g., see JP-A-2002-10267 and JP-A-2002-44669).
JP-A-2002-10267 describes a method having a procedure for limiting conversion of MPEG-2 I pictures into MPEG-4 I pictures to restrain increase in the quantity of coding generated due to direct conversion of all MPEG-2 I pictures (larger in the quantity of coding than P pictures) into MPEG-4 I pictures. Even for conversion of I pictures, the method converts the I pictures into P pictures (smaller in the quantity of coding than I pictures) on the MPEG-4 conversion side in accordance with the necessity.
JP-A-2002-44669 describes a method making use of similar correlation between motion vector information in pictures before conversion and motion vector information in pictures after conversion, that is, for performing encoding by using MPEG-2 motion vector information for decoding as MPEG-4 motion vector information. In the method, an MPEG-4 motion vector retrieving process can be omitted so that conversion time can be shortened. On the other hand, there is a video delivery system in which video data converted into MPEG-4 is not stored in a recording medium but always directly sent out to a delivery line such as the Internet (e.g., see JP-A-2002-232860). No data are held on the system side after conversion.
The method disclosed in JP-A-2002-10267 is a method in which MPEG-2 I pictures are converted into MPEG-4 P pictures in accordance with the necessity to thereby limit the quantity of coding generated in MPEG-4 as a whole. Generally, because encoding of P pictures is a process of requiring reference to other pictures, it is assumed that the time required for conversion has a tendency to increase. Hence, there is a problem that the method cannot be adapted to a system aiming at completing conversion of a great deal of video contents within a predetermined limit time.
The method disclosed in JP-A-2002-44669 is a method for performing encoding by using MPEG-2 motion vector information for decoding as MPEG-4 motion vector information to thereby shorten the conversion time. In conversion attended with conditions such as image scaling, frame skipping, etc. in the conversion process, there is however possibility that MPEG-4 may be encoded with large error because the motion vector information used is not always equal to true motion vector information. Hence, there is a problem that picture quality of video after conversion in this case deteriorates compared with that of MPEG-4 video encoded on the basis of true motion vector information. The method disclosed in JP-A-2002-232860 has a system configuration in which video contents converted in MPEG-4 are not stored. Because the method is however developed on the assumption that all MPEG-2 input frames are processed, there is no particular consideration with respect to execution of conversion of a predetermined number of video contents within a limit time. Accordingly, there is a problem that the method, like the aforementioned related-art method, cannot be adapted to a system aiming at completing conversion of a great deal of video contents within a predetermined limit time.